A display apparatus, which is one of electronic apparatuses, is used in various fields such as a personal computer, a mobile phone, a digital still camera, and a liquid crystal television. A specific example of the display apparatus is a liquid crystal display apparatus.
Such an electronic apparatus as the display apparatus includes a circuit substrate such as a TFT (thin film transistor) array substrate having, e.g., TFTs, wires, and the like.
Generally, such a circuit substrate is manufactured by repeating the following process several times. That is, a thin film is formed in accordance with a vapor deposition method such as the CVD (chemical vapor deposition) method or the sputtering method, and then an unnecessary part of the formed film is removed (etched) by way of photolithography or the like.
However, such a conventional manufacturing method suffers from the following problems (1) and (2): (1) the repeated film forming and the repeated etching cause efficiency in use of materials to be bad, and require an expensive disposal cost of many generated waste products such as an etching solution, and require long manufacturing time; and (2) immeasurable equipment cost is required for manufacture of a circuit substrate that has been demanded to be bigger in recent years. Such immeasurable equipment cost is required because the manufacture of the circuit substrate requires many vacuum apparatuses such as (i) a film forming apparatus used in each film forming step, and (ii) a processing apparatus such as an etching apparatus. A specific example of such a circuit substrate is a TFT array substrate.
In light of this, proposed in recent years is a technique for forming a wire and the like in the circuit substrate by using the “inkjet method (droplet discharging method)”. The inkjet method is a method of discharging, to a desired region, a liquid material containing conductive fine particles such that a desired pattern containing the discharged material is formed.
For example, Japanese Laid-Open Patent Publication Tokukai 2003-318192 (published on Nov. 7, 2003; corresponding to U.S. Laid-Open Patent Publication Number 2003219934; hereinafter, referred to as “patent document 1”) discloses a method for making a wire, in accordance with the inkjet method, from a paste obtained by dispersing, in an organic solvent, silver fine particles each having a particle diameter of 0.01 μm or so. The paste is an example of the liquid material, which contains the metal particles each containing any of gold, silver, copper, palladium, and nickel.
Generally, in the case of forming the circuit substrate such as a TFT array substrate used for a liquid crystal display apparatus, properties required for the wire are: (i) low electric resistance, (ii) surface smoothness, and (iii) good adhesiveness to a priming material such as glass.
However, a noble metal is generally stable, so that the noble metal has low reactivity with respect to a target (hereinafter, referred to as “application target”) to which the noble metal is to be applied. An example of such an application target is a substrate. In other words, the noble metal has poor adhesiveness to the substrate. Inkjet ink generally used in conventional techniques is “silver ink”, which is manufactured by dispersing, in an organic solvent, silver fine particles each having a particle diameter of approximately 0.01 μm. Required when silver is used as the thin film as such is: the adhesiveness to the priming material (application target) such as an application surface of a glass substrate or of an insulating material. In the sputtering, the adhesiveness of the noble metal (silver, in this case) to the substrate can be improved by an effect of striking the particles against the substrate. However, in cases where the paste using the aforementioned silver particles is printed in accordance with the inkjet method or where the paste is applied, such a striking effect cannot be expected during the film forming process. This reduces the adhesiveness to the priming material such as the glass substrate. This is not only the case with the silver particles. That is, in cases where the aforementioned metal particles has low reactivity and low adhesiveness with respect to other materials as the noble metal does, the metal particles are more easily detached from the substrate. Accordingly, the metal particles are detached with ease by a tape peel test.
Moreover, in cases where a silver film is formed on a glass substrate, grain growth is noticeable at a baking temperature of approximately 250° C. This causes a smooth surface to be rough, with the result that the surface becomes whitish. That is, such high temperature baking causes deterioration in the surface smoothness of the formed silver film, and makes it difficult for the formed silver film to be used as a wire just the way as the silver film is.
For improvement of the adhesiveness, an annealing treatment is taken into consideration. However, as is the case with the aforementioned case using the silver particles, the grain growth property of the noble metal also causes the surface of the film to be rough, with the result that the surface smoothness is deteriorated.
As such, the grain growth of the noble metal causes deterioration of the surface smoothness, and the deterioration causes various problems. See FIG. 28, for example. In FIG. 28, a lower portion wire 201 is formed by using, e.g., the aforementioned ink. The lower portion wire 201 has bad surface smoothness in a portion (cross portion (overlap portion) of wires) at which the wires overlap with one another. Such surface irregularity possibly causes short-circuit via an insulating layer 202 formed on the lower portion wire 201. In other words, such an irregularity possibly causes a defect L such as film discontinuity of the insulating layer 202. This is a cause of leakage (top-bottom leakage between upper and lower wires) between the lower portion 201 and an upper portion wire 203 formed above the lower portion wire 201, via the insulating layer 202. For example, see a case of manufacturing a TFT array substrate as the circuit substrate. In this case, in a gate electrode portion and a TFT portion, amorphous silicon (a-Si) layers (approximately 500 Å=approximately 50 nm) are so formed as to sandwich a gate insulating layer. Therefore, surface deterioration of a gate electrode serving as the lower portion wire 201 gives rise to deterioration of a TFT property, and to film discontinuity of the gate insulating layer serving as the insulating layer 202. Further, the film discontinuity caused by the surface deterioration gives rise to the aforementioned top-bottom leakage with a storage capacitor electrode. With this, the storage capacitor does not possibly work as a capacitor.